How do you start labor at 39 weeks?

well....... i am on my forth pregnancy now, i am 39 weeks, i took caster oil with my third and labour began with in 2-3 hours after taking it, but this time around i have tried it all currys, treadmill, sex, even caster oil but none of the above has worked, so i really dont think you can start yourself off, it will just happen when it is ready too, and there will be some people who will disagree but that is my theaory, as i would have swore by caster oil last time and this time it did nothing, my body just must have been ready for it then.......